Optical amplifiers may replace optoelectronic regenerators in some terrestrial and undersea long-haul transmission systems. The attraction is the potential of creating a "lightpipe" that is transparent to the bit rate and transmission format, and can convey wavelength-division multiplexed signals without the need for multiple repeaters. The difficulties associated with transmitting data long distances arise from chromatic dispersion, polarization dispersion, transmission fiber nonlinearities, optical amplifier nonlinearities and the accumulation of noise. Recent experiments have used semiconductor and erbium-doped fiber amplifiers as linear repeaters in lightwave transmission systems. The longest transmission distance reported was 2200 km. Less certain is the behavior of systems which have very long transmission distances, such as transoceanic systems operating over 7500 km.
The evolution of the signal and noise in concatenated optical amplifiers has been discussed in Quantum Electron, Vol. QE-18, No. 10 pp. 1560-1568 (1982) "S/N and Error Rate Performance in AlGaAs Semiconductor Laser Preamplifier and Linear Repeater Systems" by T. Mukai et al.; for linear optical amplifiers. However, in practice, amplifier saturation must be considered in view of the competition between the amplified spontaneous emission (ASE) and the signal for the power available from the optical amplifiers. With multiple amplifiers, this problem becomes especially severe as the ASE, which is characterized as noise, builds up in successive amplifiers to quickly cause a noise power that is higher than that of a single amplifier. In a transmission system having concatenated optical amplifiers, gain saturation of the optical amplifiers at any stage is caused by the extraction of power from the amplifier by the ASE and amplification of the signal and of the ASE from previous stages. Three methods of operating transmission systems are classified as free-running, constant-total power, and constant-signal power. Differences in performance of the three systems arise from the propagation of signal power and the buildup of the ASE.
A method for reducing the accumulation of amplified spontaneous emission noise in a system of concatenated amplifiers is needed.